The invention relates to new and useful improvements to a mounting rack for plug-in electrical printed circuit board assemblies. More particularly, the invention relates to a mounting rack for such assemblies having means for damped dissipation of electrostatic potentials.
When an electrical printed circuit board assembly is inserted into a mounting rack, external electrostatic voltages and static voltages can cause interference inside the mounting rack, and particularly to any other printed circuit board assemblies that have already been inserted. This causes an electrostatic voltage to be transferred during the insertion operation, in particular from a person holding the printed circuit board assembly to, at first, an electrically conductive front panel of the printed circuit board assembly to be inserted, especially if the front panel is metallic. The electrostatic voltages are often very high, leading to voltage discharges, such as spark discharges producing high frequency signals, or rapid rises in potential, which cause interference to electrical components on this printed circuit board assembly or to adjacent printed circuit board assemblies. This is particularly problematic where printed circuit board assemblies have to be inserted and withdrawn while other printed circuit board assemblies in the mounting rack are still operating. Insertion and withdrawal of a printed circuit board assembly during operation of the other printed circuit board assemblies is called "live insertion".
FIG. 1a shows a sectional detail of the lower region of a conventional mounting rack BGT in a side view. This figure further shows, by way of example, a section from an electrical printed circuit board assembly BG. This assembly BG has at least one printed circuit board LP with an electrically conductive front panel FP, particularly a metallic front panel, and has a connector V2 at the rear. In addition, there is a strip sliding contact K provided along the lower edge of the printed circuit board LP. This sliding contact K is conductively connected to the front panel FP of the printed circuit board assembly BG. The mounting rack BGT has at least one lower guide rail LS to assist in the insertion of the printed circuit board assembly BG in the direction ER, the guide rail being fitted between front and rear transverse rails QV, QH. At least one contact spring element F1 is provided in the front region inside the mounting rack BGT, and is electrically connected to the rack, preferably via the front transverse rail QV arranged underneath. It is advantageous for the contact spring element to be integrated in the guide rail LS. German Patent Specification DE 3 24 883 C2 describes one possible version of such an integration. When inserting the printed circuit board assembly BG, the contact spring element F1 first makes contact with the sliding contact K, producing an electrical connection between the front panel FP and the mounting rack BGT, which is normally grounded.
One disadvantage, however, is that external electrostatic voltages on the front panel FP can produce a spark inside the mounting rack BGT when contact is made between the sliding contact K and the contact spring element F1 while the printed circuit board assembly BG is being inserted into the mounting rack BGT. Such sparks produce high-frequency interference affecting, for example, operation of adjacent printed circuit board assemblies already in the mounting rack BGT.
FIG. 1b shows the same mounting rack BGT as that in FIG. 1a, but with the printed circuit board assembly BG inserted. The connector V2 at the rear of the printed circuit board assembly BG is now mated with a corresponding connector V1, the connector V1 preferably being fitted on a motherboard inside the mounting rack. External potentials applied to the front panel FP, for example electrostatic voltages from touching, flow via the sliding contact K, the contact spring element F1 and the transverse rail QV to the mounting rack BGT, which is normally grounded. However, the strip sliding contact K may act like an antenna, emitting interference deeply into the interior of the mounting rack BGT, thus once again interfering with the operation of printed circuit board assemblies that are already in the mounting rack BGT.
Finally, at least one contact screw SR is normally fitted at the bottom of the front panel FP. Once the printed circuit board assembly BG has been completely inserted, this contact screw SR is screwed, for example, to the lower transverse rail QV, thereby acting as a protective contact with an adequate current carrying capacity.